Auto Service World
Feature   June 1, 2001   by Jim Anderton

All the angles

Suspension and steering joints all contribute to the maintenance of correct steering geometry. Wear and damage to joints definitely hurt the angles.

Steering and roadholding is all about playing the angles. Steering angles, tire slip angles, and suspension geometry are all critical to safe and effective car control, whether at racing speeds or driving to the supermarket. Alignment is the primary method for reining in the many forces acting on a moving vehicle, but those forces feed into the chassis through multiple ball joints, rod ends, bearings and bushings. Wear and damage in any link in that chain can mean poor or potentially dangerous handling. And almost as importantly, it will affect customer satisfaction, especially for performance or luxury car owners.

Toe-out and toe-in are the heart of both steering geometry and alignment service, but how often have you considered lesser known alignment factors such as “Ackerman”? The answer will probably be “rarely”, but this little noticed aspect of vehicle alignment is an example of a parameter that can suffer due to steering component wear or damage.

According to Tom Byrnes, manager of product engineering for Federal Mogul (Moog):

“Ackerman Steering or Ackerman Geometry” is also referred to as “Toe out on Turns”. This is a design principal that is built into the steering geometry that states that for any given corner the outside wheel is turned on a larger radius than the inside wheel.”

With each front wheel turning at a slightly different angle, tire scrub is lessened, and steering is more positive, but the key factor from a chassis aspect is the small difference in angles. (See diagram) With as little as one degree of difference, very little damage (or moderate wear) of suspension or steering components will affect Ackerman.

“Any wear in the steering components will impact the theoretical angle. Tie rod ends, centre links, idler and Pitman arms which are worn or damaged will have a larger impact than ball joints”, declares Byrnes, who adds that the results will appear in the time-honoured fashion of alignment issues, tires: “Deviation from the theoretical angle will influence tire wear. Tire wear will first appear in the form of feath- ered edges.”

Rack and pinion systems, lacking the Pitman and idler arms, have fewer components, but are much more “reversible” than recirculating ball units. This property may feed more impact stress up into the rack and power assist componentry, rather than localizing the forces at the rod ends and linkages. The potential to feed sometimes violent forces back into the steering wheel, combined with the proven ruggedness of recirculating ball systems, however, has kept them in play in the sport utility and pickup markets long after they’ve all but disappeared from passenger cars. And in the popular light truck segment, owners perceive their vehicles as rugged and more damage tolerant than passenger cars. Four-wheel drive, high ground clearance and advertising aside, however, the same kinds of issues exist for steering components in these vehicles, with a couple of additional factors: one is that the consumer perception of toughness may lead drivers to ignore potholes and road obstacles, leading to system damage. The other is the additional stresses placed on components due to higher unsprung weight and total vehicle weight, in addition to the additional “leverage” of taller tires on ball joints, tie rod ends and wheel bearings.

Measuring the forces acting on the steering components is difficult in the lab, and very difficult in real world conditions. Federal Mogul’s Byrnes relates: “Many factors influence the magnitude and direction of the forces applied to the ball joint and tie rod end. The steering type and geometry, the speed and weight of the vehicle, and the road conditions all affect these forces. The direction of the force applied to these components is axial and radial about the stud. Depending on the type ball joint (load carrying tension, load carrying compression, or follower) and the type of tie rod end (inner or outer) these forces will vary, and can be more than double the gravitational force.”

On the street, those 2G or higher loads mean that in extreme conditions, several tons of mass are literally hanging on the stud of a ball joint. If an obstacle or pothole is encountered in a turn, a significant amount of that force is also transmitted through the steering joints.

Modern automotive steering systems are affected in similar ways, according to Jun Yoshioka, chief engineer, advanced engineering, for Dana’s Spicer Light Axle Group: “Over 90 percent of today’s cars use Macpherson strut type suspensions; if a ball joint wears, alignment is affected because it alters internal clearances. It’s not a good thing because it will give vibration feedback through the steering system. Loads are mainly inward and outward during cornering, and longitudinal forces from acceleration and deceleration. They’re resolved through the ball joint and the upper control arm.”

Engineers think of forces as vectors, imaginary arrows pointing in the direction of force, and which add up to a single “resultant” force and direction. At the bay level, it’s the result of that “resultant” that’s at issue, and that can affect geometry in ways that are imperceptible without a good alignment bench.

Damage can be easy to spot, but what about wear? Wear limits are available for most systems, but for many shops, the dial indicator is less used than the “pull and shake” method, combined with tell-tale tire wear. In terms of geometry, however, small amounts of wear can have significant effects on geometry, according to Yoshioka: “It will alter it slightly, but it will make a difference.”

Controlling wear is largely a lubrication issue, and although some aftermarket parts are equipped with grease plugs, “lubed for life” is still popular with OE manufacturers eager to minimize scheduled maintenance. “Although most of them are lubricated for life, lubrication on these components are critical”, says Yoshioka, who notes that keeping the grease in is only half the equation for long component life: “They last as long as you don’t damage the boot or get water intrusion. If you see that the boot is torn up, replace the joint. You need to watch not to damage the boot. Contamination of the grease is the first step towards failure.”

Quality replacement parts use tough synthetic rubber boots, but a “pickle fork” or errant air chisel will do damage. Replacement boots are rarely available, but even if they are, the question is, why are you there? If the knuckle is detached as part of CV, brake or steering service, it’s a good time for the customer to consider joints, too. The same reasoning applies at the rear, especially with the increasing popularity of independent rear suspension on sport utilities. At the rear, poor alignment may be more difficult for the consumer to notice, especially where highly isolated power steering systems may mask a slight pull or wander. The same holds true for binding or friction in the system. By the time the customer notices, the damage is done.

From the consumer’s view, front end (and increasingly, rear end) work is rarely cheap. It’s important, however, to resist the temptation to shave a few dollars with unproven “white box” parts.

Jun Yoshioka says: “From my perspective; I don’t want it to fail. It’s safety critical. Buying a seven-dollar joint is a gamble. An OEM-quality joint will cost more because of the manufacturing and quality materials, but it will give similar performance to the assembly line components. It’s worth spending the money. The same thought for a ball joint applies to the tie rods. It’s a free world, but I want to know where it came from.” SSGM